Earthquakes Vocabulary: Stress Strain Elastic Deformation Plastic Deformation Fault Seismic Wave Primary Wave Secondary Wave Focus Epicenter Define stress and strain as they apply to rocks. Distinguish among the three types of movement of faults. Contrast the three types of seismic waves. Faults form when the forces acting on rock exceed the rock s strength. If you bend a paperclip, it takes on a new shape. If you bend a stick, it will eventually break. The same is true of rocks; they either bend or break. Forces Within Eartth 1
Along boundaries between two tectonic plates, rocks in the crust often resist movement. Over time, stress builds up. Stress is the total force acting on crustal rocks per unit of area. When stress overcomes the strength of rocks, movement occurs along fractures in the rocks. There are three kinds of stress that act on Earth s rocks: Compression Tension Shear The deformation of materials in response to stress is called strain. Even though rocks can be twisted, squeezed, and stretched, they fracture when stress and strain reach a critical point. At these breaks, rock can move, releasing the energy built up as a result of stress. Earthquakes are a result of this movement and release of energy. 2
Under low stress, a material shows elastic deformation. Elastic deformation is caused when a material is compressed, bent, or stretched. If the stress is reduced back to zero, the deformation of the rock disappears. When stress builds up past a certain point, called the elastic limit, rocks undergo plastic deformation. Unlike elastic deformation, plastic deformation produces permanent deformation. When plastic deformation is exceeded, an earthquake occurs. When stress builds up past a certain point, called the elastic limit, rocks undergo plastic deformation. Unlike elastic deformation, plastic deformation produces permanent deformation. When plastic deformation is exceeded, an earthquake occurs. Crustal rocks fail when stresses exceed the strength of the rocks. The resulting movement occurs along a weak region in the rock known as a fault. A fault is any fracture or system of fractures along which the Earth moves. 3
There are three main types of faults that occur based on compression, tension, and shear stress. Reverse Fault Normal Fault Strike-Slip Fault Reverse faults form as a result of horizontal and vertical compression. Normal faults form as a result of horizontal and vertical tension. Strike-Slip faults form as a result of horizontal shear. Earthquake (seismic) Waves: Primary Waves Secondary Waves Surface Waves 4
The vibrations of the ground produced during an earthquake are called seismic waves. Every earthquake generates three types of seismic waves. Primary Waves Secondary Waves Surface Waves Also referred to as P-waves, primary waves squeeze and push rocks in the direction along which the waves are traveling. The compressional movement of P- waves is similar to the movement along a loosely coiled wire. (Sound also moves in compressional waves) Secondary waves, called S-waves are named with respect to their arrival times. They are slower than P- waves, so they are the second set of waves to be felt. S-waves have a motion that causes rocks to move at right angles in relation to the direction of the wave. 5
Primary and secondary waves both pass through Earth s interior. For this reason, they are also called body waves. The third and slowest type of wave are surface waves, which travel only along Earth s Surface Surface waves can cause the ground to move sideways and up and down like ocean waves. These waves usually cause the most destruction because they cause the most movement of the ground, and take the longest time to pass. The point at which seismic waves originate is the focus of the earthquake. The point on Earth s surface directly above the focus is the epicenter. Surface waves originate from the epicenter and spread out. 6
When you look in a mirror, you see yourself because light waves reflect off your face to the mirror and back to your eyes. Similarly, seismic waves travel through Earth reflect off structures inside Earth, which allows these structures to be imaged. Seismic waves can be used to make images of the internal structure of Earth. Richter scale: A numerical rating system that measures the energy of the largest seismic waves (magnitude) that are produced during an earthquake. Seismometer: An instrument that measures and records the intensity of an earthquake. Seismogram: The information recorded by a seismograph. 7
The magnitude of a wave is how much energy is released by the wave. It can be measured by the Richter Scale and the Moment Magnitude Scale. Amplitude is the height of a wave. The numbers on the Richter scale are determined by the amplitude. The moment magnitude scale is a rating scale that measures the energy released by an earthquake, but also takes in account for the size of the fault rupture, the amount of movement along the fault, and the rock s stiffness. 8
The modified Mercalli scale is a numerical system from I to XII which rates the type of damage and other effects of an earthquake as noted by observers during and after its occurrence. The intensity of an earthquake depends on the amplitude of the surface waves generated. Intensity of an earthquake decreases as the distance from the epicenter increases. Another factor that determines the intensity of an earthquake is the depth of its focus. Earthquakes can be classified as shallow, intermediate or deep, depending on the location of the focus. The majority of the world s earthquakes occur along narrow seismic belts that separate large regions with little or no seismic activity. 9
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